Gas turbines, also referred to as jet engines, are rotary engines that extract energy from a flow of combustion gas. They have an upstream compressor coupled to a downstream turbine with a combustion chamber in between. There are many different variations of gas turbines, but they all use the same basic principal.
Jet aircraft are usually powered by turbojet or turbofan engines. A turbojet engine is a gas turbine engine that works by compressing air with an inlet and a compressor, mixing fuel with the compressed air, burning the mixture in the combustor, and then passing the hot, high pressure gas through a turbine and a nozzle. The compressor is powered by the turbine, which extracts energy from the expanding gas passing through it. The engine converts energy in the fuel to kinetic energy in the exhaust, producing thrust. All the air ingested by the inlet passes through the compressor, combustor, and turbine.
A turbofan engine is very similar to a turbojet except that it also contains a fan at the front of the compressor section. Like the compressor, the fan is also powered by the turbine section of the engine. Unlike the turbojet, some of the flow accelerated by the fan bypasses the combustor and is exhausted through a nozzle. The bypassed flow is at a lower velocity, but a higher mass, making thrust produced by the fan more efficient than thrust produced by the core. Turbofans are generally more efficient than turbojets at subsonic speeds, but they have a larger frontal area which generates more drag at higher speeds.
Turboprop engines are jet engine derivatives that extract work from the hot-exhaust jet to turn a rotating shaft, which is then used to spin a propeller to produce additional thrust. Turboprops generally have better performance than turbojets or turbofans at low speeds where propeller efficiency is high, but become increasingly noisy and inefficient at high speeds.
Turboshaft engines are very similar to turboprops, differing in that nearly all of the energy in the exhaust is extracted to spin the rotating shaft. Turboshaft engines are used for stationary power generating plants as well as other applications.
One problem associated with gas turbine engines, especially in aircraft, is the possibility of flame-out, which occurs when the flame becomes extinguished within the combustion chamber. One of the causes of flame-out is instability of the flame front within the combustor. Since engine failure during flight is clearly problematic, it would be advantageous to construct a gas turbine engine such that the possibility of flame-out was reduced. For stationary power generating systems there is a need for reduced emissions, primarily NOx, in order to meet newer, more stringent clean air requirements.
Additionally, power burners of various types have been in use for many years. “Nozzle mix” or “gun style” burners are those burners that inject fuel and air separately in some manner so as to provide a stable flame without a ported flame holder component. Other types of power burners use some method of pre-mixing the fuel and air and then delivering the fuel-air mixture to a ported burner “head”. These “heads” or “cans” can be made of a variety of materials including perforated sheet metal, woven metal wire, woven ceramic fiber, etc. Flame stability, also referred to as flame retention, is key to making a burner that has a broad operating range and is capable of running at high primary aeration levels. A broad operating range is desired for appliances that benefit from modulation, in which the heat output varies depending on demand. High levels of primary aeration are effective in reducing NOx emissions, but tend to negatively impact flame stability and potentially increase the production of Carbon Monoxide (CO). High levels of primary aeration (also referred to as excess air) also reduce appliance efficiency. There is a need in the art for a fuel burner that reduces the production of NOx while maintaining flame stability. Even more desirable is a burner that produces very low levels of NOx while operating at low levels of excess air.